Iron chelation promotes 5-aminolaevulinic acid-based photodynamic therapy against oral tongue squamous cell carcinoma

Abstract

Background: Oral tongue squamous cell carcinoma (OTSCC) is the most common malignancy of the oral cavity. Photodynamic therapy (PDT) has become a clinically promising approach for early stage OTSCC treatment. 5-Aminolaevulinic acid (5-ALA) is a precursor of protoporphyrin IX (PpIX) and has been applied for PDT of cancer. However, the accumulated PpIX in 5-ALA-treated cancer cells will be further transformed into heme through ferrous iron insertion under ferrochelatase catalysis. Theoretically, iron chelation can enhance the intracellular accumulation of PpIX and thus promote 5-ALA-based PDT. Here, an iron chelator deferasirox (DFX) was used to investigate synergistic suppression effects of 5-ALA-based PDT and iron chelation on OTSCC.

Methods: In OTSCC SCC-25 cells, the enhancing effect of DFX on 5-ALA-mediated accumulation of PpIX was firstly assessed. After laser irradiation (635 nm, 200 mW/cm² and 2 min), the synergistic cytotoxicity and apoptosis-inducing effect of 5-ALA and DFX were evaluated in SCC-25 cells, and the apoptosis mechanism was further investigated by monitoring the change of mitochondrial membrane potential and observing the subcellular localization of cytochrome c (Cyt c). In SCC-25 tumor-bearing mice, the synergistic suppression effects of 5-ALA-based PDT and DFX on tumor growth and tumor angiogenesis were investigated after laser irradiation on the tumor (635 nm, 150 mW/cm² and 10 min).

Results: In SCC-25 cells, DFX showed strong iron chelation effect and enhanced 5-ALA-mediated intracellular accumulation of PpIX by 2-3 folds. After laser irradiation (635 nm, 200 mW/cm² and 2 min), 5-ALA combined with DFX exhibited significant synergistic effects on cytotoxicity and cell apoptosis. In the treated cells, the damage of mitochondrial membrane and the release of Cyt c from mitochondria to cytoplasm were observed distinctly, indicating the activation of mitochondria-related signal pathway. In SCC-25 tumor-bearing mice, tumor growth and tumor angiogenesis were both notably suppressed by combination treatment of 5-ALA with laser irradiation and DFX. Meanwhile, no obvious toxic injuries were visible in histological examination of major organs in the treated mice.

Conclusions: 5-ALA-based PDT combined with iron chelation synergistically inhibited the growth of OTSCC. Hence it can be seen that this combination therapy may represent a promising strategy for clinical treatment of OTSCC and other cancers.

Keywords: 5-Aminolaevulinic acid; Deferasirox; Iron chelation; Oral tongue squamous cell carcinoma; Photodynamic therapy.